elm elm_object_item: Fixed item part content set hooks.

elm elm_object_item: Fixed item part content set hooks.
1. delete old content object
2. check whether new content object is same as old content object
3. rename internal content set hook function
4. elm_widget_sub_object_del will be followed by evas_object_del
5. added some doxygen description.

git-svn-id: svn+ssh://svn.enlightenment.org/var/svn/e/trunk/elementary@69738 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33